Typical electrophotographic (EP) devices have a spinning polygon mirror that directs a laser beam on a photoconductor, such as a drum, to create one or more scan lines of a latent to-be-printed image. As is known, multiple scan lines extend in a same direction, e.g., left-to-right, and provide a common referencing of all scan lines relative to a single laser beam sensor position, known typically as a horizontal synchronization (or “hsync”) position. Often, the hsync signal is defined in units of time for the engine of the EP device and its apparent location exists in a space somewhere off the edge of the printed page.
However, it has recently been suggested that torsion oscillator or resonant galvanometer structures can replace the traditional spinning polygon mirror. In this manner, scan lines can occur in both the forward and backward directions (e.g., bi-directionally) thereby increasing efficiency of the EP device. Because of their MEMS scale size and fabrication techniques, the structures are also fairly suggested to reduce the relative cost of manufacturing. Unfortunately, scanning in two directions adds complexity to image referencing since two reference points need occur at opposite ends of the printed page and even the slightest of deviations between scan lines amplifies print image imperfections. Also, EP device parameters, such as beam sensor signal delays, optical component alignment, and galvanometer or oscillator scan profile nonlinearity, must be measured and accounted for.
For instance, a laser beam is swept across an hsync sensor to generate an electrical pulse, according to individual characteristics of the sensor, when the beam has passed over a sufficient amount of the sensor with a sufficient amount of light. From when the beam actually first impinges the sensor to the time when the actual electrical pulse is sent, a delay of varying amount occurs. Also, by the time the EP device has processed the pulse, the actual location of the beam has changed.
If this delay were always a fixed amount, then calibrating the sensors would be fairly intuitive. However, it is not fixed and varies according to process conditions, such as the amount of optical power in the laser beam, the spot size of the beam, the temperature of the hsync sensor, and the sensitivity of the hsync sensor.
Accordingly, there exists a need in the art for very accurately knowing the starting location of a scan line for bi-directionally scanning EP devices to improve print quality. Particularly, there are needs by which knowing the starting location relates to characterizing the manner in which a laser beam transits an hsync sensor and correlating the same to electrical pulses sent from or asserted by the sensor. Ultimately, a need further exists to use the foregoing to align the forward and reverse bi-directional scans. Naturally, any improvements should further contemplate good engineering practices, such as relative inexpensiveness, stability, low complexity, ease of implementation, etc.